Presently, there are two methods of roasting coffee, a hot blast method and a heat radiation method. The hot blast method is easily utilized in a coffee roaster, as shown in U.S. Pat. Nos. 4,325,191 and 4,271,603. However, the hot blast method produces coffee which is sour and does not smell, or taste, like real coffee. On the other hand, the heat radiation method maintains the smell and flavor of coffee. Yet the construction of the roaster for the heat radiation method is complicated and it takes a long time to roast the coffee.
A roasting drum is normally cylindrically shaped and comprises a cylindrical drum element and a bottom plate. Either the drum element or the bottom plate is operatively connected to a driving source to provide a rotating motion, and the remaining part is fixed on the apparatus. Because one element is fixed and the other is rotating, a gap exists between the two elements. If the gap between the drum element and the bottom plate becomes too large, the coffee beans may fall into the gap and finally interfere with the rotating motion. Also, if the gap between the drum element and the bottom plate is too small, the friction between the drum element and the bottom plate will interfere with the smooth rotational motion.